Method and means of producing a large diameter single-crystal rod from a polycrystal bar

ABSTRACT

A method and means for producing a large diameter monocrystalline or single-crystal rod of semiconducting material from a polycrystalline bar comprising the supporting of the bar at an angle with the vertical and the heating of the lower end to a molten state by means of a high-frequency induction coil disposed thereabout. The rod may be formed by passing a monocrystalline seed through a space in the induction coil to contact the molten end of the bar forming a fused junction and retracting the seed downwardly while rotating it to draw the molten bar material between the coils out of the heated area to form a recrystallized rod. An additional heating coil may be positioned beneath the angularly disposed bar and heating coil to provide a slow and controlled heat recession during the solidification of the rod and the rod may be gas-doped by introducing the gas into the area of the molten zone.

United States Patent [72] Inventor Emil R. Caplta FOREIGN PATENTS 7020Kennedy Blvd North Bergen, J- 1,061,745 1 1/1957 Germany 23/273 0704?1,482,659 6/1966 France 23/301 [21] Appl. No. 696,560 [221 Filed hm 9,1968 Primary ExamrnerNorman Yudkoff I 45] Patented Sept 21 1971Assistant Examiner-S. Sllverberg Attorney-Norman N. Holland [54] METHODAND MEANS OF PRODUCING A LARGE zi gggiifik ROD FROM A APSTRACT: A methodand means for producing a large 2 Claims, 2 Drawing as. d1am etermonocrystallme or s1ngle-crystal rod of semiconductlng material from apolycrystalline bar comprlslng the U-S- SP, upporting of the bar at anangle the vertical and the 23/301 SP heating of the lower end to amolten state by means of a high- 1 Int. frequency induction coildisposed thereabout The rod may be O SP, formed passing a monocrystanineseed through a pace in 273 SP; 29/2531252/629 the induction coil tocontact the molten end of the bar forming a fused 'unction andretracting the seed downward] while [56] References Cited rotating it todraw the molten bar material between tl ie coils UNITED STATES PATENTSout of the heated area to form a recrystallized rod. An addi- 2,962,36311/1960 Martin 23/273 SP tional heating coil may be positioned beneaththe angularly 2,972,525 2/1961 Emeis 23/273 SP disposed bar and heatingcoil to provide a slow and controlled 3,002,824 10/1961 Francois...23/301 SP heat recession during the solidification of the rod and therod 3,036,892 5/1962 Siebertz.... 23/301 SP may be gas-doped byintroducing the gas into the area of the 3,477,811 11/1969 Keller 23/301SP molten zone.

METHOD AND MEANS OF PRODUCING A LARGE DIAMETER SINGLE-CRYSTAL ROD FROM APOLYCRYSTAL BAR BACKGROUND OF THE INVENTION '1 ne present inventionrelates to an improved method and apparatus for manufacturingsemiconductor materials and more particularly for manufacturing a largediameter monocrystalline or single-crystal rod from a bar ofpolycrystalline material.

The term bar herein shall be used exclusively in connection with thesupply of unmelted polycrystalline material and the term rod shall beused in connection with the supply of resolidified monocrystallinematerial merely in an attempt to avoid confusion between the two, and inno way should these terms be interpreted to indicate the relativediameters or other dimensional features of these members.

Two well-known methods of manufacturing semiconductors of materials suchas silicon and germanium involve the melting of a supply of the materialin polycrystalline fomi, contacting the molten material with amonocrystalline particle or seed and retracting the seed to draw or pullthe molten material with it, thereby resolidifying the melted materialand producing a semiconductor material in monocrystalline form. Theearliest and a still popular method consists in the melting of thematerial in a heated crucible, and the clipping of the seed therein.However, an alternate method was subsequently developed wherein a bar ofthe material is held only at its ends preferably in a vertical positionand a molten zone produced in the bar extending through its crosssection. This molten zone is gradually passed along the bar byrelatively moving the energy source, which produces the heat to melt thezone, and the bar parallel to the bar axis. The molten zone is freelysupported by the adjacent nonmelted material of the bar. This operationis commonly performed to purify the material causing foreign substancesto become dissolved and collect at the ends of the bar material.

This zone melting method is adapted to the production of monocrystals ifthe end of the polycrystalline bar is placed into contact with amonocrystalline rod, or merely a piece or seed of the same materialarranged in a suitable crystal orientation, and the molten zone isproduced first at the junction point and then gradually and successivelycarried through the polycrystalline bar. The bar material crystallizesat the rear side of the molten zone in monocrystalline condition.

The zone melting method is also particularly conducive to the doping ofsemiconductor rods with the desired impurities. The doping substancesare introduced into the molten zone and then become part of therecrystalizing semiconductor material at the rear side of the moltenzone. This method produces a material of a higher degree of purity andan undisturbed monocrystal formation as there is little possibility ofundesirable impurities diffusing into the molten zone such as from thewalls of a vessel which may occur during the crucible melting operation.However, the zone melting method has been technologically more difficultto carry out as the freely floating liquid material may easily drop off.

A further disadvantage of the zone method is the limitation on the sizeof the monocrystalline rod which can be produced. In present practicethe bar of material to be melted is generally maintained in the verticalposition and the heating means such as a high-frequency induction coilsurrounding the bar is moved up and down along its length to produce themolten zone; or, alternatively, the coil is fixed and the bar movedthrough it. In either event, the use of this coil prevents significantenlarging of the diameter of the resulting monocrystalline rod as thepossible diametrical enlargement of the recrystal lized rod over that ofthe bar is limited by the diameter of the heating coil. In addition,with this method a high degree of heat is required to melt the barcompletely through to the core to insure that the entire zone ofmaterial becomes molten and subsequently recrystallized. The presentinvention is designed to overcome these various problems presented bythe prior method of zone melting and is adapted to permit the resultingmonocrystalline rod to be of a much larger diameter than the bar to bemelted while requiring less heat to be used in the process.

SUMMARY OF THE INVENTION The improved method and means of the presentinvention comprises the arranging of the axis of the polycrystalline barto be melted, at an angle with respect to the axis of themonocrystalline seed. This permits the end of the bar to be heated by aninduction coil disposed thereabout, insuring melting of the entire crosssection of the bar. Also, the coil elements may be spaced, permittingthe molten portion of the bar end to pass through the space in theinduction coil before resolidifying. With this arrangement it ispossible to construct the monocrystalline cord of any desired diameter,as the limiting dimensions of the heating coil no longer interfere withthe formation of the resolidified monocrystalline material.

It is therefore an object of the present invention to provide a methodand means for producing a monocrystalline rod from a polycrystalline barwithout restricting the relative diameters of the members.

It is another object of the present invention to provide a zone methodof producing purified semiconducting material, which requires the use ofless heat than the prior art methods.

It is another object of the present invention to provide an improvedmethod and means for producing purified monocrystalline semiconductingmaterial, permitting more rapid and improved melting than achieved bythe prior art methods.

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative embodiment about to be described, orwill be indicated in the appended claims, and various advantages notreferred to herein will occur to one skilled in the art upon employmentof the invention in practice.

BRIEF DESCRIPTION OF DRAWINGS A preferred embodiment of the inventionhas been chosen for purposes of illustration and description and isshown in the accompanying drawings, forming a part of the specification,wherein:

FIG. 1 shows the molten end of the polycrystalline bar being contactedby a monocrystalline seed in accordance with the present invention.

FIG. 2 is a showing as in FIG. 1, after the monocrystalline seed hasbeen retracted to form a portion of the resolidifled monocrystallinerod.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I shows a bar ofpolycrystalline semiconducting material 1 having a high frequencyinduction heating coil 2 disposed at its end. One or more of the coilelements 3 may be arranged to surround the bar 1 and at least one of theelements 4 is disposed directly beneath the end. This arrangementinsures heating of the bar material throughout the entire cross sectionat the end, resulting in the melting of a zone covering the entire tipof the bar. It will be seen that less heat is required with thisarrangement to melt the bar through its entire cross section than isrequired by the prior art method of heating about the lateralcircumference of the bar to produce the molten zone.

A monocrystalline seed 5 of the same material as the polycrystalline bar1 is held in a crystal holder chuck 6, which is mounted by means of aslip clutch 7 on a shaft 8 that may be rotated and controlled in up anddown movement. A space 9 is provided between the coil elements 3surrounding the bar 1 and the elements 4 disposed beneath its end topermit the monocrystalline seed 5 to pass therethrough and contact theend of the bar 1. It will be noted this operation is performed whilearranging the axis of the polycrystalline bar 1 at an angle with respectto the axis of the seed 5 and shaft 8. It has been found to bepreferable to maintain the seed and shaft axis in a vertical directionwhile tilting the bar axis from the vertical, as the flow of the moltenmaterial is assisted by gravity in this orientation. However, it iswithin the purview of the present invention to orient the particularelements in various other angular arrangements to essentially accomplishthe same results.

An additional heating coil is disposed beneath the end of the bar 1 andcoil 2 to provide a slow and controlled heat recession during theresolidification process as the molten material passes therethrough, asshown in FIG. 2.

The method of the present invention is performed in the followingmanner. The end of the polycrystalline rod 1 is preheated to produce theinitial molten condition. This may be accomplished by inserting atantalum disc in the area so that the radiated heat will cause thepolycrystalline material to become conductive to a point where the highfiequency power will take over an induce into the material directly.

When the end of the bar 1 is completely melted the monocrystalline seed5 is passed up through the space 9 in the coil elements 3 and 4 andcontacts the molten lower edge ,of the bar 1. This contact results inthe formation of a fused junction 11 between the two materials. Duringthis operation the shaft 8, including the seed 5 and clutch 7 arerotating. After a suitable junction has been achieved, additional moltenmaterial will be caused to flow into this melted area. The seed 5 isthen slightly retracted and the bar 1 is proportionately fed to thejunction area so as to build up a cone-shaped portion of resolidifiedmaterial, as shown at 12 toward the bottom of FIG. 2. An up-and-downmotion of the shaft 8 while feeding the bar material 1 is continueduntil the desired rod diameter is obtained. Thereafter the shaft 8 islowered at a predetermined rate and the bar 1 is fed at a predeterminedrate to produce a uniform diameter resolidified monocrystalline rod 13on the end of the shaft 8.

The bar 1 may also be rotated in the opposite direction from that of theshaft 8 while performing the method, to keep the molten zone in rapidrotation, thereby providing a thorough mixing of the material therein.This motion may also contribute to increasing the diameter of therecrystallized rod 13 when desired. The seed-holding chuck 6 is mountedon the shaft 8 by means of the slip clutch 7 to provide a release in theevent of a freezeup.

While the above operation can be performed in a high vacuum or an inertgas atmosphere, this method is particularly adapted to achieve improvedgas doping of the semiconducting material. As shown in FIG. 2, thedoping gas 14 may be introduced into the area between improved heatingcoils 2 and 10 and the reduced cross section of the molten material atthis point permits the gas to be more evenly diffused throughthematerial in the resolidified rod 13. The gas may also be directedagainst the lower surface of the molten end of the bar with similarresults. The lower heating coil 10, which as previously stated, providesa slow and controlled heat recession apparatus. In addition, improvedgas doping may be achieved with the arrangement.

As various changes may be made in the form, construction and arrangementof the parts herein and in the steps of the method herein withoutdeparting from the spirit and scope of the invention and withoutsacrificing any of its advantages it is to be understood that all matterherein is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim: 1. An. apparatus forproducing a monocrystalllne rod of semiconducting material from apolycrystalline bar comprismg:

a. means for supporting the bar at an acute angle with the vertical withat least one end free;

b. spaced high frequency induction coils about and directly beneath thefree end respectively to heat the free end to a molten state; therespective coils being in planes parallel to each other;

' c. means for placing a vertically oriented monocrystalline seedbetween the respectively spaced induction coils to contact a molten edgeof the bar at its lowest extremity; and

d. means for retracting the seed along a vertical axis thereby drawingthe molten bar material out of the heated area to form a recrystallizedrod.

2. An apparatus for producing a monocrystalline rod of d. means forretracting the seed along a vertical axis thereby drawing the molten barmaterial out of the heated area to form a recrystallized rod; and

e. additional high'frequency coils adjacent to the molten zone toprovide a slow and controlled heat recession during the solidificationof the rod.

2. An apparatus for producing a monocrystalline rod of semiconductingmaterial from a polycrystalline bar comprising: a. means for supportingthe bar at an acute angle with the vertical with at least one end free;b. spaced high frequency induction coils about and directly beneath thefree end respectively to heat the free end to a molten state; c. meansfor placing a vertically oriented monocrystalline seed between therespectively spaced induction coils to contact a molten edge of the barat its lowest extremity; d. means for retracting the seed along avertical axis thereby drawing the molten bar material out of the heatedarea to form a recrystallized rod; and e. additional high-frequencycoils adjacent to the molten zone to provide a slow and controlled heatrecession during the solidification of the rod.